The gas-phase clustering reactions of protonated amino acids, AAH(+) (AA = Ala, Val, Ser, Phe, Pro, Gln, Lys and Arg) with acetonitrile were studied with a pulsed ion-beam high-pressure mass spectrometer. The bond energies of the AAH(+)center dot(CH3CN) complexes range from 20 to 11 kcal/mol. While for AlaH(+), ValH(+), SerH(+), PheH(+), ProH(+) and GlnH(+), the solvation free energies decrease linearly with increasing gas-phase basicity of AA, deviations from the correlation are observed for LysH(+) and ArgH(+). The bonding free energies for AAH(+)center dot(CH3CN) are compared with those for AAH(+)center dot(H2O) and AAH(+)center dot(NH3). (C) 2010 Elsevier B.V. All rights reserved.